Photovoltaic solar cell

ABSTRACT

A photovoltaic cell comprising, a cathode layer, an electron donating layer, and an electron accepting anode structure comprising a collection region including one or a plurality of secondary collection regions attached to the collection region. The secondary collection regions of the anode preferably are in the shape of dispersed rod shaped branches extending within a continuous phase of the electron donating layer. The collection region is preferably located at a central point with respect to the secondary collection regions, to thereby provide a photovoltaic cell structure with improved charge collection and efficiency of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 10/308,722, filed Dec. 3, 2002, which claimspriority to U.S. Provisional Application Ser. No. 60/337,265, filed onDec. 5, 2001.

FIELD OF INVENTION

The present invention relates to photovoltaic cells and a method ofproducing such photovoltaic cells with improved charge collection andefficiency.

BACKGROUND OF THE INVENTION

In the past twenty years research activity has increased dramatically inthe field of conductive polymers. The reason was the discovery thatconjugated polymers can behave as metallic conductors andsemiconductors. These polymers did not gain commercial significancebecause they were not stable. In the last five years work hasconcentrated on increasing the environmental stability andprocessability of these polymers. See, M. C. Gallazi et. al.“Regiodefined Substituuted Poly(2,5-thienylene)s, J. Poly. Sci. Part A:Polymer Chemistry, Vol. 31, 3339-3349 (1993); G. Zotti et. al. “Novel.Highly Conducting, and Soluble Polymers from Anodic Coupling ofAlkyl-substituted Cyclopentadithiophene Monomers”, Macromolecules 1994,27, 1938-1942; and K. J. Ihn et. al. “Whiskers ofPoly(3-alkylthiophene)s, J. Poly. Sci. Part B: Polymer Physics, Vol. 31,735-742, (1993). In addition, it has been reported that the addition ofside chains have rendered the various polymers more soluble and havetherefore also stabilized, to some extent, their structure.

More recently, various papers have alluded to the advancement insemi-conducting polymer technology that are used as charge separators toproduce photo-induced electron transfer. See, e.g., J. H. Burroughes et.al. “New Semiconductor Device Physics in Polymer Diodes andTransistors”, Nature, Vol. 335, 8 Sep. 1998; R. N. Marks et. al. “ThePhotovoltaic Response in Poly(p-phenylenevinylene) Thin-Film Devices” J.Phys.: Condens. Matter 6 (1994); C. W. Tang Appl. Phys. Lett., Vol. 48,No. 2, 13 Jan. 1986; J. H. Burroughes et. al. Nature Vol. 347, 11 Oct.1990 pp. 539-541; N. C. Greenham et. al. Chem Phys Letters 241 (1995)89-86; and G. Yu et. al. Appl Phys lett. 64 (25)20 Jun. 1994. These newmaterials provide “[a] molecular approach to high efficiencyphotovoltaic conversion” G. Yu et. al. The efficiency with which thesepolymers convert photons to electrons is near 100%. However, the overallefficiency of the cell is hindered by inefficient collection of thecarriers. See, G. Yu et al. J. Appl. Phys. 78 (7), 1 Oct. 1995. N. C.Greenham et. al. reported “[t]he problem of transport of carriers to theelectrodes without recombination is a more difficult one to solve, sinceit requires that once the electrons and holes are separated ontodifferent materials, each carrier type has a path way to the appropriateelectrode without needing to pass through a region of the othermaterial”. See, Phy. Rev. B, Vol. 54, No. 24, 1996, pp. 17628-17637.This problem appears to be the roadblock to continuing progress in thisfield.

G. Yu et. al. (J. Appl. Phys, 78 (7), October 1995) and J. J. M. Hallset. al. (Nature, Vol. 376 (1995) 4510) suggested the problem can bealleviated by phase separation of the two charge carriers, therebycausing the photoinduced reaction at the donor/acceptor (D/A) interfaceto occur at the boundary between the two phases while allowing theseparate carriers to migrate through their own phase (see FIG. 1). Thissolution offered some increase in efficiency (˜2%), however, thedisorder in the phase separated regions did not allow easy collection ofthe carriers.

A. J. Heegar (TRIP Vol. 3, No. 2, February 1995) attempted to increasethe order in the phase separated blends by creating a network of onepolymer in the other. The increase in efficiency was only marginal(˜2.3%). The closest attempt to create the ideal structure wasaccomplished by B. O'Reagn et. al. (Nature, Vol. 353 (1991) 737) andU.S. Pat. No. 5,084,365. They accomplished it by using a mixture of dyesand nanometer sized titania particles. The resultant cell gave an energyconversion efficiency of 12%. The reason for this significant increasewas due to the large surface area afforded by the nano particles inclose proximity to the charge transfer couple material. In order to getthis, however, they had to use liquid electrolyte that seeped into the“nooks and crannies” of the porous anode to establish an electricconnection with the light active sites and harvest the holes that werecreated by them. The porous nature of the anode created the large D/Asurface area while the liquid electrolyte acted as the hole collector inthis case. In addition collection was done through an electrolyte viaion charge transfer rather than actual hole transport. The above workdemonstrated that with an efficient collection scheme the overall energyconversion efficiency could be dramatically improved. The above cellhowever, is not conducive to large area roll to roll manufacturing,since it used liquid as one of the components in the cell.

Since then many researchers have tried to replace the liquid electrolytewith various solid electrolytes and other solid hole conductor materialswith little success (a few % efficiency). See, e.g., Kei Murakoshi et.al. Chemistry letters, 1997 p. 471-472; A. C. Arango et. al. Mat. Res.Soc. Symp. Proc. Vol. 561 pp. 149-153; A. C. Arango et. al. AppliedPhys. Letters Vo. 74, No. 12 pp. 1698-1700; K. Yoshino et. al. IEEETrans. Elec. Dev 44 (8), p. 1315-1323 (1997); and R. N. Marks et. al. J.Phys Condens. Matter 6, p. 1379-94 (1994); and T. J. Savenje et. al.Chem Phys. Lett. 290, p. 297-303 (1998). The conclusions always returnsto the same basic problem: to avoid recombination losses the layers needto be made very thin which in turn diminishes light absorption and hencecharge generation. Alternatively, if the two materials are intermixedcharge generation is high, but, collection is severely repressed due toa high recombination rate. Charge collection remains the key challengefor high efficiency and hence the commercialization of polymeric basedsolar cells.

To optimize the efficiency, and avoid the above problems of the priorart, an object of the present invention is to develop a photovoltaiccell wherein one electrode is connected to all the separate donor phasesto collect the holes and another electrode is connected to all theindividual acceptor regions to collect the electrons, thereby resultingin a high efficiency photovoltaic cell.

More specifically, it is an object of the present invention to developthe above referenced photovoltaic cell wherein rods of one phase (theelectron accepting anode) are placed inside the matrix of the otherphase (i.e., the electron donating cathode). Hence charge separationwould be made to occur at the interface between the rods and the matrixwhile charge transport would take place through the rod anodes for onecarrier and through the matrix cathode for the other.

Finally, in accordance with the objectives of the present invention, ameans of making an electrical contact to all the electrical acceptinganode rods in the matrix needs to be established. Such method thenprovides the means to allow charge collection into a single point i.e.to the electrode.

The present invention also has as its object the development of astructure to be used as an efficient means of collecting free electronscreated by charge separation from photon excitation of a donor/acceptorpair. This configuration will then allow electron collection through thecentral point collector in the acceptor phase and hole collection viathe electrical contact of the donor phase with a metal conductor layer.

SUMMARY OF THE INVENTION

A photovoltaic cell comprising a cathode layer, an electron donatinglayer, an electron accepting anode structure comprising a centralcollection region including one or a plurality of secondary collectionregions attached to said collection region. An additional electrondonating layer may be included to thereby provide an electron donatinglayer on both sides of the electron accepting anode structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a charge generation and transport in a donor/acceptortype solar cell.

FIG. 2 illustrates the use of electron accepting rods in a matrix typeconfiguration.

FIG. 3 illustrates preferred configurations for the electron acceptingrods of the present invention.

FIG. 4 illustrates a partially broken away isometric view of thepreferred embodiment of the photovoltaic cell of the present invention.

FIG. 5 is a representational drawing of a continuous web manufacturingline for production of a block copolymer based photovoltaic cell of thepresent invention.

FIG. 6 illustrates a web with solar cells produced via the printingmethod of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A two phased system, created to achieve an efficient charge collectionphotovoltaic cell, wherein one of the species (the acceptor) is moreelectronegative than the other specie (donor) in a two phased system.The polymeric system is further present in a preferred controlledmorphology comprising acicular (e.g., rod) shaped domains of theacceptor phase in a host matrix of the donor phase. The acicular shapeddomains of the acceptor phase are preferably further present in a treelike arrangement within the continuous matrix of the donor phase. Theacceptor phase is preferably comprised of a central collection point andmany branch like structures that emanate from it.

In addition, it is preferable that with respect to the preferredphotovoltaic cell of the present invention: (i) the electron acceptinganode structure is disposed in the electron donating layer; (ii) theelectron accepting anode is more electronegative than said electrondonating layer; (iii) the electron accepting anode structure comprisesTiO₂; (iv) the electron accepting anode structure comprises TiO₂ andconductive particles; (v) the conductive particles may comprise silveror gold; (vi) the electron accepting anode structure comprises an N-typesemi-conductor material; (vii) the electron accepting anode structurecomprises an N-type semi-conductor material and conductive particles.

The present invention is further directed at a method for producingpolymeric based photovoltaic cells using continuous web manufacturingtechniques, wherein a photovoltaic active polymeric system is applied toa substrate, acicular shaped domains of one of the components of thesystem are arranged into a tree like geometry, and the substrate ismetallized, all in a roll to roll type manufacturing process.

As shown in FIG. 4, the basic structure of a preferred polymeric basedphotovoltaic cell, shown generally at 10, is a layered structurecomprising three layers: a metallic (cathode) layer 12, a photovoltaic(PV) active layer 14, which is preferably polymer based, and a substrate16, wherein all three layers are preferably present as an integralstructure. As shown in FIG. 4, the photovoltaic cell 10 also includes anegative electrode 18 which preferably penetrates into PV layer 14.Electrode 18 penetrating into PV layer 14 is the anode of photovoltaiccell 10, with the circuit completed by metallic layer 12 serving as thecathode for photovoltaic cell 10.

In a polymeric based photovoltaic cell of the present invention, PVlayer 14 is the active photovoltaic component. Therefore, in addition tothe preferred structure of photovoltaic cell 10, the operation of thephotovoltaic cell is to a degree dependent upon the composition andmicrostructure of PV layer 14. The photovoltaicly active system iscomposed of two species, and while both of the species of the system aresemi-conducting, one species is preferably more electronegative than theother specie, herein referred to the acceptor specie and the donorspecie respectively.

Furthermore, in the polymeric based photovoltaic cell the acceptorspecie is preferably an N-type semi-conductor and the donor specie isP-type semi-conductor. The difference in electronegativity of the twospecies is a measure of the electron affinity of each specie, whereinthe acceptor specie has a greater electron affinity than the donorspecie. In addition to the relative electron affinities of the twospecies, the donor specie is selected from a group of semi-conductingpolymers that are capable of inducing a charge separation uponexcitation by electromagnetic radiation, that is the material isphoto-conducting. Useful charge separation additionally prefers that thedonor material has a high absorption in the visible spectrum ofelectromagnetic radiation.

In addition to the chemical characteristics of the species of thesystem, the morphology of PV layer 14 is preferably a network ofinterconnected branches of the acceptor specie in a continuous matrix ofthe donor specie. The network of branches preferably collect at acentral point 24, a structure termed an electrical tree 22, asillustrated in FIG. 3. It is understood, however, that there are severalalternatives to the the electrical tree configuration 22. For example,such alternatives may comprise a leaf type pattern, fractal pattern,etc.

Electrode 18 of photovoltaic cell 10 is positioned in PV layer 14 at apreferred central collection point 24 of electrical tree 22, asillustrated in FIG. 4. Electrical tree 22 extends, preferably, in aradial pattern from anode electrode 18 and is contained within PV layer14.

In operation, a charge separation is induced in PV layer 14 at theinterface of the rod domains of the acceptor material and the donorcontinuous phase. The free electrons are transported in the branches ofelectrical tree 22 to anode electrode 18, from which point theelectrical flow can be utilized. The electrical circuit is completed bythe return of the electrons to the donor continuous phase throughmetallic layer 12, serving as the cell cathode.

Provided that the conductive requirements of the species of PV layer 14are satisfied, the morphological requirements of the system may besatisfied by several different alternatives. One way, as noted, is forthe acceptor specie to be present as rod shaped domains in the donorcontinuous phase which are subsequently oriented to produce aninterconnected structure. Another method is to electrochemicallypolymerize and disperse the acceptor specie in situ. Yet another methodis to actually print the acceptor specie in the shape of a tree using astandard press with the acceptor material as the ink.

Yet another method involves the use of a blend of incompatiblehomopolymers, or block copolymers with incompatible blocks, in which theblocks or homopolymer blend comprise the acceptor polymeric specie andthe donor polymeric specie. In either case, the proportion of theacceptor polymer specie to the donor polymer specie is controlled toachieve the preferred dispersed rod shaped domains of the acceptorpolymeric specie in a continuous matrix of the donor polymer specie.

In the preferred embodiment substrate 16 is polyester film, however,substrate 16 may be selected from any material which can be metallizedin a standard web metallization process. Alternate materials forsubstrate 16 include, but are not limited to, polycarbonate,polystyrene, acrylic, etc. Anode electrode 18 is preferable a high workfunction material including, but not limited to, gold, silver, copper,and carbon.

The layer structure can also be reversed in its construction. Top layer14 can be coated directly onto the substrate followed by the treestructure 22 and a second layer 14. The cell is then completed by acathode metal layer that is preferably vacuum coated as the top layer inthis arrangement. Depending on ease of manufacture, either configurationwill function as a high efficiency photovoltaic cell.

The basic photovoltaic cell 10 can further be improved by the additionof non-reflective coating to top layer 14 or the bottom layer of thesubstrate depending on which configuration is chosen. The use of suchnon-reflective coatings will prevent the reflection of incident light,therein allow a greater quantity of light to proceed to PV layer 14. Itis also possible to dope the preferred active photovoltaic polymersystem with a conductive material to increase the overall electricalconductivity of PV layer 14. However, when selecting a dopant, both thetype of dopant and the quantity used should be preferably controlledsuch that not only is the dopant generally uniformly dispersed in PVlayer 14, but also to ensure that the morphology of the preferredpolymer system is not adversely affected, i.e. the acceptor specieremaining in the preferred form of an elongated shaped domains arrangedin a preferred electrical tree configuration. While meeting the aboverequirements, the dopant may be added to the polymer system either insolution, or during a compounding step prior to dissolving thepolymer(s).

As a preferred process for manufacturing polymeric based photovoltaiccells, the present invention relies upon a continuous web roll to rolltechnique. There are several roll to roll methods that can be employedin the fabrication of the above cells. In the case of morphology controlof both incompatible polymer systems as well as the case ofelectrochemical polymerization methods, a modified atmospheric rollcoater is preferably used. By way of example, FIG. 5 depicts a roll toroll coater for the block copolymer system. In the figure, substratefilm 16 is supplied from a source roll 26, and is supported by a numberof rollers and idler wheels (omitted for clarity), and is finallycollected on a take up roll 28, thereby creating a continuous web ofsubstrate 16. Substrate film 16 is a metallized polymeric film (e.g.copper on Mylar). It is coated with a solution of a PV from a standardcoating apparatus such as a Gravure, slot die, etc. 34. The PV solutionis coated such as to leave exposed edges 36 and 38 of the metallizedlayer. The PV solution on substrate film 16 dries as the web continuesto move. As drying progresses, the PV system phase separates into rodshaped domains of the acceptor polymer specie dispersed in a continuousdonor polymer mix.

While the PV solution is drying an electric field is applied tosubstrate film 16, therein orienting the conducting polymer rod shapeddomains perpendicular to the direction of the electric field. Substratefilm 16 contains conductive edges 36 and 38 along which metallic idlerwheels 40 and 41 travel. A high voltage power supply 44 is connected toeach of the idler wheels 40 and 42 to bias the metal layer underneaththe PV coating, and therein orient conductive rod shaped domains. Whenthe PV solution has substantially dried, the PV layer is flooded withhigh energy electrons from electron beam unit 46. The now charged PVcoating is discharged using a grounded rotating pin wheel 48, whereinthe pin wheel 48 is rotated by the moving web. The grounding pins 50 onpin wheel 48 preferably comprise a high work function metal whichintermittently make contact with the coated web at predeterminedlocations, both transversely and longitudinally, on moving substrate 16,thereby discharging the PV layer and forming an electrical treedischarge pattern. Subsequent to discharge of the web, a protective topcoating is applied to the PV layer. The top layer preferably hasanti-reflection properties. The top coating is applied usingconventional techniques, for example using a sputtering apparatus 52.After the PV layer has been top coated, the web is collected to take uproll 28.

In an alternate preferred embodiment, after discharging the PV layer,the polymer layer may be completely dried and collected on take up roll28 without a top coat. The metallizing process may be carried out as asecondary process, as before, using sputtering or other conventionalcoating methods.

While the above process is described in terms of continuous webmanufacturing, the preferred manufacturing steps require only that thePV is coated onto a substrate from solution, thereby allowing thepolymer system to self-assemble into rod shaped domains of the acceptorpolymeric specie dispersed in a continuous matrix of the donor polymericspecie. The rod shaped domains are then ordered into an electrical treearrangement by charging the PV layer and then discharging it through asingle point.

The same coater as above can also be used for an alternative method ofproducing the patented structure using an electrochemical polymerizationprocess instead of orienting block polymers. The PV coating is madeinstead, of fully polymerized and end terminated donor polymers alongwith monomers of the acceptor specie. The metallized undercoat is biasedpositive to 3 Volts. Polymerization of the acceptor monomers willproceed from the metallized layer. Polymer chains will grow in a randomfashion from the metallized layer, and produce tree like structures allover the metal surface and imbedded in the donor polymer phase.Subsequent to polymerization of the acceptor chains and drying of thedonor phase, a second metal layer is deposited onto the PV layer toserve as the cathode. The latter metallic layer preferably comprises alower work function metal than the metal layer that is deposited ontothe substrate. The metallic coating is applied using conventionaltechniques, for example using a sputtering apparatus 52. After the PVlayer has been metallized, the web is collected to take up roll 28. Noelectron beam unit 46, nor grounding pin 48 are required for thismethod.

The third preferred process capable of producing the present inventioncell structure is a standard printing press (i.e. offset lithography,letter press, flexography etc.). The substrate is metallized by aconventional web metallization process (e.g. vacuum evaporation,sputtering). The acceptor and donor species are dissolved or dispersedin a vehicle to be made into an ink suitable for the type of printingpress chosen. The press should ideally have four printing towers orstations. The first tower is used to print a uniform pattern (i.e. floodcoat) of the donor polymer in 4″-8″ wide strips (or a few strips acrossof less width if the press is wide enough) leaving 2″ gaps of exposedmetal surface of the substrate between the strips. The next towerpreferably prints a small square (2″×2″) every 4″ or so in the exposedarea (i.e. in the unprinted gaps). The ink is a standard insulatingvarnish (e.g. UV ink, polyurethane, acrylic etc.).

The next tower then prints an image of a tree made of ink that containsthe acceptor material. The last tower puts on a similar flood coat asthe first tower using an ink also containing the donor polymer but withthe addition of an environmentally stable binder (e.g. polyurethane).The pattern created by this optional method is depicted in FIG. 6. Itshows a repeating tree pattern “sandwiched” between two layers of thedonor material. The insulating pad isolates the anode and cathode fromeach other (i.e. the tree from the metallized substrate).

Based upon the foregoing disclosure, it shall be understood that thepresent invention has a variety of options and alternatives, all fallingwithin the scope of the various claims, appended hereto.

1. A system, comprising: a cathode layer; an electron donating matrix;and an anode comprising a network comprising a plurality ofinterconnected acicular branches, the anode being disposed within theelectron donating matrix; wherein the system is configured as aphotovoltaic cell and the photovoltaic cell is devoid of an additionalanode structure.
 2. The system of claim 1, wherein the anode issupported by the electron donating matrix.
 3. The system of claim 1,wherein the network comprises a primary collection branch and theplurality of interconnected acicular branches are connected to theprimary collecting branch.
 4. The system of claim 3, wherein the primarycollection branch comprises a material selected from a group consistingof gold, silver, copper, and carbon.
 5. The system of claim 1, whereinthe network is in a tree configuration.
 6. The system of claim 1,wherein the anode comprises an N-type semiconductor material.
 7. Thesystem of claim 1, wherein the anode comprises an N-type semiconductormaterial and conductive material.
 8. The system of claim 1, wherein theanode comprises TiO₂.
 9. The system of claim 1, wherein the anodecomprises silver or gold.
 10. The system of claim 1, wherein the anodecomprises a polymer comprising an electron acceptor domain and theelectron donating matrix comprises an electron donating polymercomprising an electron donating domain.
 11. The system of claim 10,wherein the electron acceptor domain and the electron donating domainare incompatible.
 12. A system, comprising: a cathode layer; and aphotoactive layer comprising a continuous electron donating matrix andan electron accepting network disposed within the continuous electrondonating matrix; wherein the electron accepting network is configured asan anode and comprises a primary branch comprising a conductivematerial, the system is configured as a photovoltaic cell, and thephotovoltaic cell is devoid of an additional anode structure.
 13. Thesystem of claim 12, wherein the electron accepting network comprises aplurality of interconnected acicular branches.